Video: Understanding how overprints and knockouts work

Okay, so we have started to get a little bit more of an appreciation of the fact that there is little bit more to transparency that meets the eye. You know, it's more than just blend modes or Opacity settings, but as we've seen already that inside the Transparency panel, we can use the Isolate Blending setting to make sure that elements within a group don't mix with elements that are outside of that group. Now there's another setting here called Knockout Group. However, before I can explain to you exactly what that setting does and how we might be able to use it in a real world situation, we first have to really get a good understanding of what a knockout is in general.

In this installment of Illustrator Insider Training, Mordy Golding shows experienced Illustrator users how to create transparency effects and ensure reliable printing results. This course reviews the history of vector transparency and covers features such as knockout groups, opacity masks, and transparency flattening. Mordy also shows how to establish a safe workflow when placing Illustrator graphics containing transparency in PostScript, PDF, and InDesign files. A free worksheet is included with the course.

Understanding how overprints and knockouts work

Okay, so we have started to get a little bit more of an appreciation of the factthat there is little bit more to transparency that meets the eye.You know, it's more than just blend modes or Opacity settings, but as we've seenalready that inside the Transparency panel, we can use the Isolate Blendingsetting to make sure that elements within a group don't mix with elements thatare outside of that group.Now there's another setting here called Knockout Group.However, before I can explain to you exactly what that setting does and how wemight be able to use it in a real world situation, we first have to really get agood understanding of what a knockout is in general.

So let's take a look now for a moment on this file right here.It's called flower.ai and it's very straightforward.I'm working right now in a CMYK document and I've simply taken two objects here,a blue flower, which looks just like that.I'm going to press Command+Z to move it back. And then a yellow circle thatappears in the middle of it.So I have these two elements.Again, one is filled 100% yellow.One is filled 100% cyan.Now in order to get a better understanding of exactly what knockout is inside ofIllustrator, I'm going to employ two other panels inside of Illustrator.

I'm going to go to the Window menu and I'm going to choose to open upmy Attributes panel.We're going to see I have the ability to apply something called an overprint to my artwork.I'm also going to go back to the Window menu and I'm going to choose to open upmy Separations Preview panel, which will allow me to view color separations onmy screen right here inside of Illustrator.Now I'll do one other thing here inside of Illustrator as well.I'm going to go over to my View menu and turn on this setting herecalled Overprint Preview.This will allow me to preview overprints on my screen again here inside of Illustrator.

So let's see exactly what this Knockout is and how regular artwork gets printedinside of any document.Now I have two objects here inside of my document and the yellow object issitting on top of the blue object that appears beneath it.Now let's for a moment understand exactly how printing works.I'm going to print out separations out of this, meaning I'm going to get a platethat is purely cyan and I'm going to have a plate that's purely yellow.This is now going to go through a printing press and first we're going to printthe cyan color and then we're going to print the yellow color on top of it.

Now if I print the cyan, the blue part first, and then I put the yellow on top of that,I overlay the yellow on top of the blue, then the ink on the paper itself isgoing to mix in the area where the yellow and the blue overlap each other.When you mix yellow and blue, you get green.So that means that when I actually print this out on a printing press, this isgoing to look like a blue flower with a green circle in the middle, not a yellow one.So what normally happens with regular artwork inside of Illustrator is that whenI create separations, artwork that appears on top of other artwork knocks outthe artwork that appears beneath it.

In other words, Illustrator is automatically going to remove any blue thatappears beneath the yellow circle.We can actually see this happening.I'm going to go here now to my Separations Preview panel and I'm going to turnoff Cyan, turn off Magenta, and turn off Black.So all I'm viewing right now is the yellow plate itself.This is what's going to print out on the yellow plate. That actually won't printin the color yellow, but I'm right now previewing this using yellow so I can get inmy head a better idea of what this is going to look like.The plate itself is actually completely solid black and then that printer justsimply uses yellow ink for that plate.

However, this is the actual artwork that appears printed on the yellow plate itself.Now I'm going to turn on the Cyan plate.I'm going to turn off the Yellow plate, then you can see that on the cyan platethere is no ink that's going to print on the middle part here of this flower.That's because the yellow object that appeared on top of this knocked out orremoved any artwork that appeared beneath it.This allows me to on a printing press get a blue flower with a yellowcenter, because now there's no mixing of the yellow and the blue inks on the paper itself.

Let me turn on the CMYK plates here, the complete composite here, and let'sgo for a moment now.I'm going to select just this yellow object right here and in my Attributespanel I'm going to set that object to Overprint.When I do so, I'm overriding Illustrator's default behavior,the default behavior of knocking out objects that appear beneath other artwork.By choosing Overprint Fill I'm instructing Illustrator to actually keep the bluebackground beneath the yellow one.And because I have Overprint Preview turned on right now, I'm now seeing asimulation of what this is going to look like when printed on a printing press.

Because now the blue flower does not have that middle area knocked out.Again, to show you what that looks like on the plates itself, I'm going to turnoff Cyan, Magenta, and Black.The Yellow plate looks exactly the same.Let me deselect it so it's easier to see.But if I now turn on the Cyan plate and turn off the Yellow plate, you can seethat now there's no white circle.There's no knockout that's created in this plate.So it's just important to understand here the terminology.When you have regular artwork that is stacked on top of each other,those elements on the top of the stacking order by default will always knockout theobjects that appear beneath it, meaning it'll remove the artwork that appearsdirectly beneath it.

However, I can choose to override that behavior by telling objects to overprint.When I tell an object to overprint,that means that it should not remove the fill beneath it and that way I willcombine these two elements on press.This is kind of a poor man's way of actually achieving more colors than you'reactually willing to pay for on press.Just to give you an example, if these were two Pantone colors,just say blue and yellow here in this case, and I only wanted to pay for printingtwo colors on a press, I could simulate actually three colors by using thesetwo colors, but then specifying that wherever these colors overlap, they alsocreate the illusion of a third color, because I'm actually mixing those two colors together.

In fact, if I turn this on, it looks like that yellow circle is transparent. It's not.It's just set to overprint so that those two colors actually mix on press.But again, the important thing here really that I just kind of want to drivehome is this terminology.We know something called overprinting, but we also know somethingcalled knocking out.And again knockout means the artwork that appears underneath is actually removed.Keep that in mind, because in the next movie we're going to learn exactly whatthe Knockout Group setting does in the Transparency panel.

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